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Drug Industry Starts Race to Develop Zika Vaccine

Success could be years away; Sanofi hopes to gain from its work with dengue virus

As many as 1.5 million Brazilians may have been infected with the mosquito-borne Zika virus and now the U.S. and other countries are also reporting new cases. But what is the Zika virus? And why does it pose a threat to pregnant women? Dipti Kapadia explains. Photo: Getty Images

PARIS—Drug companies are beginning early-stage research to develop a new vaccine against the rapidly spreading Zika virus, joining the race to control an outbreak that the World Health Organization said constitutes a global public-health emergency.

It might be years, however, before any vaccine reaches the market, meaning the new wave of research is unlikely to help curb the current outbreak. Though Zika generally makes people only mildly ill for a few days, it has been linked to a serious birth defect in which babies are...